Dispersion, refers
to an optical property of gemstones whereby flashes and pinpoints of
spectral colors are displayed as the stone is turned in the light.
The dispersive colors we see are not really there in the gem,
instead, they are created by the behavior of white light in the stone.
Dispersion results when light passes through a transparent material
with inclined surfaces (like a prism, or a faceted gemstone).
Although the term "fire" is gemologically equivalent to dispersion,
"fire" is so frequently misused to mean either brilliance (total
light return) or scintillation (twinkling), that I prefer to use
"dispersion" in all my descriptions for the sake of clarity.

[Visible light range of the electromagnetic
spectrum]

White light is, of
course, made up of a spectrum of wavelengths from relatively long
(red) to relatively short (blue and violet). Each of these
wavelengths is bent to a different degree (red less, blue more) when
passing from air into a denser medium like a gemstone. When the bent
light waves exit through an inclined surface (like a facet),
depending on the degree of bend (or refraction), they may show as
distinct spectral colors. The ability of a gem species to show
dispersion is, therefore, roughly correlated with the density and
refractive index of the gem material itself.

This property is a
distinctive characteristic of each gem species and can be used in the
process of identifying a gem. Testing for dispersion in gems,
however, is actually a rather painstaking and complicated process
involving measuring the separate refractive indices of the red and
blue wavelengths in that species and calculating their ratio.
Instead, most gemologists and gem lovers simply gauge dispersion by
eye.

There are
published tables (see below) of the laboratory values for each
species, but in actuality several other factors may enhance or
depress the display in a given stone. Foremost among these is body
color. For example, two species with high values for dispersion:
demantoid garnet and Benitoite tend to have fairly dark body color
which usually masks the effect to a great degree. Fans of dispersion
and fans of rich color often part company over which is more
beautiful--> a saturated medium dark blue Benitoite which shows
little of its potential dispersion, or a substantially lighter one
with spectral colors flashing at every turn. (As a card carrying
Benitoite fancier, I am definitely in the latter category!) The 2.0
ct. Benitoite stone in this pendant has, to my eye, an optimal
balance between color and dispersion.

[Beniotite showing dispersion]

The two
sphalerites below show the effect of darker body color on dispersion,
the yellow stone showing much more than the darker orange
stone.

[Yellow
sphalerite, orange sphalerite]

In general, the
larger, cleaner and lighter in color the gem, the more of its
potential dispersion will be visible. Cutting style has a noticeable
effect as well, in that higher crowns accentuate the effect and
flatter crowns diminish it. (I once cut a large, light lavender
spinel (a species with modest dispersion figures) with an extremely
tall crown that had more "fire" than a lot of poorly cut diamonds I
have seen.

Diamonds have
always been admired for this property, and so diamond simulants have
been sought that have similar dispersive characteristics. Up until a
few decades ago, the diamond simulant of choice was white zircon,
whose dispersion and high luster make it a good visual replica for
diamond.

[Diamond, white zircon]

Synthetic rutile
and strontium titanite were each briefly popular when first
synthesized, but were noticeably more dispersive than diamond and so,
not very convincing.

[Synthetic rutile, strontium titanite]

YAG (yttrium
aluminum garnet), was also used, but with dispersion noticeably less
than diamond, it was unsatisfactory as well.

[YAG]

Cubic zirconia now
has the lion's share of the simulant market, among other reasons,
because its dispersion, though higher than diamond is close enough to
look right, especially in sizes most commonly used in jewelry.

[Cubic
zirconia, CZ]

In the
list below are a few species that might be seen in today's
marketplace that typify gems with low, moderate, high and very high
dispersion values: